These studies have focused on: (1) the relation of the quality of medical care to the risk of perinatal death, (2) the tendency to repeat similar birth weight and gestational age in subsequent pregnancy outcomes to the same mothers, (3) perinatal mortality in relation to order of birth and size of sibship, (4) epidemiologic risk factors for preterm birth, (5) epidemiologic risk factors for small-for-gestational age births, (6) contribution of multiple births to perinatal mortality rates, (7) size at birth as measured by head circumference, crown-heel length, and birth weight in relation to gestational age and perinatal mortality.